1. Field of the Invention
The present invention relates to a permanent magnet which is compact and has a high performance, to a motor including a rotor provided with a the permanent magnet, and to a method for magnetizing the permanent magnet for the motor.
2. Background Information
In recent years, computers have been put in widespread use in a rapid pace. Disc-like storage media such as a hard disc and a magneto-optic disc are widely used for storage devices in most of those computers. Those storage media are rotated at a high speed by a small-sized motor provided to a disc drive and data is read or written from and to the media.
The motor used for the disc drive is designed to generate a rotating magnetic field by means of a stator coil so as to act on a permanent magnet provided to a rotor, thereby rotating.
The permanent magnet is magnetized radially (in a radial direction) and inverts polarities at a predetermined pitch along a circumferential direction. That is, the N pole and the S pole appear at regular intervals along an inner or outer circumference. The same polarity is attained in the same pitch; the number of domains where the same polarity appears is referred to as the number of poles.
At present, most widely used ones are motors that excite a stator coil and rotate a permanent magnet having 12 poles using three-phase AC. Regarding the permanent magnet size, the magnet having an outer diameter of about 25 (mm) has been dominantly used.
Those permanent magnets are magnetized by means of a large magnetic field generated by a magnetizing head installed to the inner or outer circumferential surface of the permanent magnet.
A lead wire is provided inside the magnetizing head, which allows a pulsed DC (several tens of thousands of amperes (A)) to flow therethrough.
As regards a permanent magnet for an outer rotor type motor, the magnetizing head is arranged at an inner circumferential portion of the permanent magnet to magnetize the magnet. This is because the outer rotor type motor has a stator coil that is arranged toward the inner circumferential surface of the permanent magnet, which requires high polarization property (definite border between the N pole and the S pole) of the inner circumferential surface of the permanent magnet. In short, the magnetizing head and the inner circumferential surface of the permanent magnet are arranged face to face. As a result, the polarization property of the inner circumferential surface of the permanent magnet is improved.
In contrast, in the case of using a permanent magnet for an inner rotor type motor, the stator coil is arranged to face the outer circumferential surface of the permanent magnet, so the magnetizing head is provided at an outer circumferential portion of the permanent magnet to magnetize the magnet.
Most of those permanent magnets are rare earth magnets. Of the rare earth magnets, a bonded magnet formed of an Nd—Fe—B based material as an isotropic magnetic material has been used in a great many cases.
The term isotropic means that the directions of magnetic force of the magnetic material extend not in the same direction but randomly. Magnetizing the material yields a magnet.
Also, the term bonded magnet means a magnet obtained by mixing a resin with a magnetic material for facilitating molding of the magnetic material.
In addition, in recent years, as disclosed in JP 2000-195714 A, an Sm—Fe—N bonded magnet has come into use.
Then, various attempts have been taken to realize a multipolar magnet making use of anisotropic property of the Sm—Fe—N bonded magnet or to prepare an Sm—Fe—N bonded magnet resistant to rust; an Nd—Fe—B bonded magnets is inferior in resistance to rust.
Here, the term anisotropic means that the magnetic force in the constituents of the magnetic material is generated in the same direction. A magnet with a high magnetic force, as compared with the one obtained by magnetizing the isotropic magnetic material, can be obtained by previously aligning the directions of the magnetic force and then magnetizing the material.
A hard disc has been recently used also in other fields than a computer field, for example, a digital camera, car navigation system, or information appliance. Along with this tendency, a small-sized motor for driving a memory disc of a storage device with a size of typical 2.5 inches or smaller has been growing in the market.
In response to the recent application of the small-sized hard disk top rotatable devices such as the digital camera or information appliances, there are demands for a downsized, energy-saving motor that realizes a high torque.
Downsizing the motor leads to reduction in size of the permanent magnet used therefor. Then, downsizing the permanent magnet necessitates reduction in size of a magnetizing head for magnetizing the permanent magnet.
When the magnetizing head is downsized, a lead wire used therefor is thinned. A direct current supplied to the magnetizing head is reduced, in part, because of the withstand voltage of the magnetizing head. As a result, there is a problems that full magnetization (magnetic saturation) for the permanent magnet is not always allowed and the ability inherent in the magnetic material cannot be completely brought out.
It is therefore an object of the present invention to provide a permanent magnet which is compact and has a high performance, a motor equipped with the permanent magnet, and a method for magnetizing the permanent magnet.